(1) Field of the Invention
The present invention relates to methods and apparatus for cooling substrates, such as semiconductor wafers, glass substrates for photomasks, glass substrates for liquid crystal displays or substrates for optical disks. Specifically, substrates having been heated to a high temperature are placed on or adjacent a cooling plate having a cooling device to be cooled to a target temperature that is approximately room temperature before and after a step of photoresist application or development during a photolithographic process.
(2) Description of the Related Art
A conventional example of substrate cooling apparatus of the type noted above is illustrated in FIG. 1. This apparatus includes a treating chamber 100 housing a heat conduction plate 101, Peltier elements 102 and a radiating plate 103 constituting a cooling plate assembly 104, a plurality of substrate support pins 105 arranged to be vertically movable through perforations 104a formed in the cooling plate assembly 104, and an air cylinder 106 for raising and lowering the substrate support pins 105. For cooling a hot substrate or wafer W, a substrate transport robot (not shown) places the wafer W on the substrate support pins 105 in a raised position. Then, the support pins 105 are lowered so that the wafer W rests on the cooling plate assembly 104. At this time, the Peltier elements 102 are driven under proportional-plus-integral-plus-derivative (PID) control or the like whereby, as shown in the time chart of FIG. 2, the temperature Tp of cooling plate assembly 104 is maintained equal to target temperature Ts to which the wafer W is to be cooled.
However, with this substrate cooling apparatus, as shown in FIG. 2, when hot wafer W is placed on the cooling plate assembly 104 at cooling starting time t1, plate temperature Tp is temporarily raised by wafer temperature Tw. At this time, the Peltier elements 102 of the cooling plate assembly 104 are subjected to PID control to become target temperature Ts. However, since there is a limitation to the cooling capacity of Peltier elements 102, it is inevitable that the cooling plate assembly 104 is temporarily heated above target temperature Ts. Thus, a long time tp is consumed before plate temperature Tp returns to target temperature Ts, which in turn requires a long time tw for cooling the wafer W to target temperature Ts. This results in the disadvantage that the apparatus has low throughput.